Reporting multiple sets of CSI, including Precoding Matrix Indicator (PMI), Channel Quality Indicator (CQI), and Rank Indicator (RI) as well as Precoding Type Indicator (PTI) and Point Indicator (PI), could enable higher scheduling flexibility at eNode B (eNB) and hence improve cellular system performance, referring to 3GPP (3rd Generation Partnership Project) TS 36.213 V10.5.0, which is incorporated hereto by reference. For example, FIG. 1A and FIG. 1B schematically show two assumptions for CQI calculation in a heterogeneous deployment. In FIG. 1A and FIG. 1B, a user equipment (UE) 103 is located in a cell covered by a eNode B (eNB) 102 and receives Physical Downlink Shared Channel (PDSCH) from the eNB 102. The cell covered by the eNB 102 is overlapped with a macro cell covered by another eNB 101. When the UE 103 reports CSI to the eNB 102, two sets of CQI can be selected. One set of CQI is calculated by assuming the macro cell is not muted, that is, there is interference from the eNB 101 indicated by the dashed arrow, as shown in FIG. 1A, and another set of CQI is calculated by assuming the macro cell is muted as shown in FIG. 1B. Conventionally, a semi-static configuration of multiple CSI reporting is used as shown in FIG. 2. FIG. 2 schematically shows a conventional multiple CSI reporting, in which each block represents one subframe. In the conventional multiple CSI reporting, two sets of CSI are both reported periodically, one of which is PMI1, CQI1 and RI1 and is for example calculated by assuming the macro cell is not muted as shown in FIG. 1A, and the other of which is PMI2, CQI2 and RI2 and is for example calculated by assuming the macro cell is muted as shown in FIG. 1B. RI1 and RI2, which will be referred to as long term CSI components in the context herein, can be reported less frequently, and PMI1, CQI1, PMI2, and CQI2 will be referred to as short term CSI components in the context herein. For example, RI1 and RI2 are reported every 200 subframes, while PMI1/CQI1 and PMI2/CQI2 are respectively reported every 5 subframes as shown in FIG. 2. Since both sets of the short term CSI components are reported frequently and periodically, the reporting of multiple sets of CSI increases overhead in uplink due to the consumption of much more subframes taken by the frequently reported short term CSI components.
In order to reduce overhead but still report different sets of CSI, which are for example calculated based on different assumptions in the heterogeneous deployment as shown in FIG. 1A and FIG. 1B, one possibility is to allow a UE to dynamically select a set of CSI from multiple possible sets of CSI to be reported. For example, if a UE is in Cell Range Extension (CRE) region, the UE may switch between the two CQIs. When the UE is allowed to select a set of CSI to be reported, the UE also needs to report an additional Transmission Scheme Assumption Indicator (TSAI) to inform the eNB of which set of CSI is selected by the UE, for example, in the illustrated heterogeneous deployment shown in FIG. 1A and FIG. 1B, to inform eNB which assumption is used by the UE to calculate the values of the CSI being reported. In addition, considering that the interference condition may change dynamically in cellular system, the preferred downlink (DL) transmission scheme at the eNB may also change dynamically. Therefore, it is preferred to report TSAI dynamically, which implies that TSAI should be reported together with PMI/CQI, for example, in the same frames, as shown in FIG. 3.
FIG. 3 schematically shows the CSI reporting with TSAI and with only a single RI being reported. In FIG. 3, the long term CSI component RI is reported for example every 200 subframes, and then the short term CSI components PMI/CQI and TSAI, which are encoded in the same subframe, are reported for example every 5 subframes. Different from FIG. 2, in which both sets of PMI/CQI are reported every 5 subframes respectively, in other words, there exist two series of periodical reporting in FIG. 2, in FIG. 3, only one series of PMI/CQI is reported, the UE determines which set of PMI/CQI will be reported each time, and a TSAI is reported together with the PMI/CQI to indicate which set of PMI/CQI is selected at that point. As exemplarily shown in FIG. 3, a first set of PMI/CQI together with a first TSAI (PMI1/CQI1/TSAI with hatched background) is chosen to be reported directly after the reporting of RI (which is RI1 in this example), and after four subframes, the first set of PMI/CQI and the first TSAI are still chosen to be reported, and then after another four subframes, a second set of PMI/CQI together with a second TSAI (PMI2/CQI2/TSAI with dotted background) is chosen to be reported. However, the choosing of which set of PMI/CQI being reported each time is not limited to the above described. The UE can choose any one of multiple possible sets of short term CSI components to report each time according to interference conditions, and the TSAI is used to inform the eNB which set is selected.
In FIG. 3, only one single RI (RI1 in this example) is reported since the same RI is enforced for different sets of PMI/CQI. However, it is noted that for different TSAI, the optimal value of RI can be different. Therefore, if the same RI is enforced for different sets of PMI/CQI, optimal values of RI can not be reported, and such restriction may cause severe performance degradation in some scenarios. In order to solve this problem, a straightforward solution is to report RI dynamically, i.e., with the same frequency as PMI/CQI, but in a different subframe, as shown in FIG. 4. FIG. 4 schematically shows the RI reporting with the same periodicity with TSAI. Different from the measure of FIG. 3, the long term CSI component RI is reported each time the short term CSI components PMI/CQI are reported in FIG. 4. In this case, the optimal values of RI can be reported; however, such solution would cause much overhead and occupy many uplink subframes due to that RI is reported as frequently as short term CSI components, and also increase collision probability.